Automatic blood pressure monitors are commonly used to periodically measure the blood pressure of a patient. In most automatic blood pressure monitors, a pressure cuff is attached to a patient's arm over the brachial artery. The cuff is first pressurized with an applied pressure that is high enough to substantially occlude the brachial artery. The cuff pressure is then gradually reduced, either continuously or in increments. As the pressure is reduced to systolic pressure, the flow of blood through the brachial artery beneath the cuff increases substantially.
When the blood flows through the brachial artery following each contraction of the heart, it imparts a pulsatile movement to the wall of the artery. This pulsatile movement is coupled to a blood pressure cuff extending over the artery as minute changes in the cuff pressure, which are known as oscillometric pulses. Automatic blood pressure monitors employing the oscillometric method measure and record the amplitude of the oscillometric pulses at a number of cuff pressures. After the blood pressure measurement had been completed, a table contains the oscillometric pulse amplitudes recorded at each cuff pressure.
In theory, the systolic, diastolic, and mean arterial blood pressures can then be determined from the values in the table using theoretical and/or empirical definitions of these parameters as a function of the amplitudes of these oscillometric pulses. However, blood pressure measurements are often adversely affected by artifact, generally produced by patient movement. Motion-induced artifact can substantially alter the measured amplitude of oscillometric pulses thus introducing inaccuracies in the measurement of the patient's blood pressure.
The use of "signal averaging" is a conventional technique to extract periodic signals in the presence of random noise. It is used in many fields, both medical and non-medical. Within medicine, signal averaging is most often used to extract neural evoked potentials. A number of commercial blood pressure monitors average an attribute of the oscillometric pulse, usually pulse amplitude, to eliminate artifacts. For example, the averaging of oscillometric peak amplitudes to replace a value judged to be artifact is mentioned in U.S. Pat. Nos. 4,754,761 and 4,638,810 to Ramsey, 4,799,492 to Nelson, and 4,190,886 to Sherman.
A number of conventional devices use the QRS-complex of the ECG to help eliminate artifacts from blood pressure measurements. The QRS-Complex is the portion of the ECG that represents the contraction of the ventricles of the heart. Most of these devices use a technique called "ECG-Gating". By ECG-Gating, blood pressure signals are accepted only when they appear in a specified temporal relationship to the QRS complex. None of these prior art devices use the QRS-Complex to average the input data. Moreover, these prior art devices using ECG-Gating are auscultatory rather than oscillometric. Auscultatory methods for blood pressure measurement rely upon the detection of the Korrotkoff sounds just as the physician depends upon these sounds when he or she uses a stethoscope.
U.S. Pat. No. 4,860,759 to Kahn mentions the QRS-Complex in describing a non-invasive blood pressure monitor. However, the QRS-Complex is not actually used by the monitor in connection with making blood pressure measurements. Kahn does use a pulse sensor located on a finger distal to the blood pressure cuff to determine blood pressure. However, averaging techniques are not used. The pulse sensor merely detects blood flow through the cuff just as a physician uses Korrotkoff sounds to indicate that blood flow.
U.S. Pat. No. 4,974,597 to Walloch discloses a blood pressure monitor that uses the QRS-Complex to detect artifacts. Again averaging techniques are not used. The QRS-Complexes are used to bracket the time period during which a single oscillometric pulse or Korrotkoff sound is expected.
U.S. Pat. No. 4,677,984 to Shramek discloses using the time between the QRS-Complex and the detection of a pulsatile pressure change beneath the cuff to reconstruct the waveform of the inter-arterial pressure wave. Again averaging is not involved.
It has not heretofore been realized that useful oscillometric waveforms can be extracted from artifact by averaging all of the data points of the oscillometric waveform rather than just a single attribute of that waveform.